Polyether polyols useful in the production of polyurethane products generally have a high proportion of propylene oxide-derived oxypropylene moieties, and are prepared conventionally by the base catalyzed oxyalkylation of a suitably functional initiator molecule such as propylene glycol, glycerine, trimethylolpropane, sucrose, or the like. The propylene oxide used to prepare such polyols is subject to a competing rearrangement to allyl alcohol under the reaction conditions, as discussed in detail in BLOCK AND CRAFT POLYMERIZATION, Ceresa, Ed., John Wiley and Sons, New York, pp. 17-21. The monofunctional allyl alcohol species generated by the rearrangement competes with the desired initiator molecule and its oxyalkylated oligomers for propylene oxide, resulting in the presence of polyoxyalkylene monols in addition to the desired di-, tri-, or higher-functionality, initiator-derived polyoxyalkylene polyols. As the allyl alcohol species continues to be generated as the reaction proceeds, the mole percentage of monol continues to increase, and as a practical matter, polyol equivalent weights greater than c.a. 2000 Da are difficult to achieve. Even at these equivalent weights, the mole percentage of monol may approach 50 mol percent, and the theoretical functionality lowered from the nominal value of 2, in the case of a polyoxypropylene diol, to average functionalities approaching 1.5. The monofunctionality may be determined from measuring the unsaturation content of the polyol product, or by GPC analysis. As each monol contains a point of ethylenic unsaturation, titrametric measurement of the unsaturation is generally recognized as reflective of monol content.
Despite the drawbacks associated with base catalyzed oxypropylation, catalysts such as sodium and potassium hydroxide, and to a lesser extent the corresponding lower alkoxides, continue to be used commercially. At present, base-catalyzed polyols constitute the vast majority of commercially available polyether polyols for urethane applications. In addition to the previously described problems, basic catalyst residues must be removed from the polyol product. Both neutralization with acids, as well as use of solid adsorbents such as magnesium silicate have been used in the latter respect. However, the nature of the polyurethane reaction makes it in general very sensitive to catalyst residues, and many cases have been documented where the substitution of a neutralized polyol for one purified by use of an adsorbent or vice versa has caused polymer system failure.
In the decades of the '60s and early '70s, a new class of oxyalkylation catalysts based on double metal cyanide complexes such as the non-stoichiometroc glyme complex of zinc hexacyanocobaltate were developed. These catalysts were found to enable preparation of higher molecular weight polyoxypropylene polyols having much lower levels of unsaturation than base-catalyzed analogs. See, for example, "Hexacyanometalate Salt Complexes As Catalysts For Epoxide Polymerizations", R. J. Herold et al., ADVANCES IN CHEMISTRY SERIES, No. 128, .COPYRGT. 1973, American Chemical Society and Herold, U.S. Pat. No. 3,829,505, which disclose unsaturation in the range of 0.015 to 0.020 meq/g polyol achieved through the use of double metal cyanide complex catalyzed oxyalkylation. These references further disclose polyurethane foam preparation from freshly prepared polyether triols containing 240 ppm catalyst residues.
Although the presence of such large amounts of residual double metal cyanide catalyst residues in polyether polyols may not influence foam properties when used shortly after preparation, the storage stability of such products has been questioned due to production of volatile components over time. As polyether polyols destined for use in polyurethanes are often stored for extended periods, storage stability represents a significant commercial problem. See, e.g., J. L. Schuchardt et al., "Preparation of High Molecular Weight Polyols Using Double Metal Cyanide Catalysts", 32ND ANNULAR POLYURETHANE TECHNICAL MARKETING CONFERENCE, Oct. 1-4, 1989; and Herold et al., U.S. Pat. No. 4,355,188. Moreover, presence of double metal cyanide catalyst residues have been stated to result in increases in isocyanate prepolymer viscosity during storage, perhaps due to allophanate formation. See, e.g., Schuchardt, op. cit., and Herold '188 op. cit. As a result, numerous methods have been devised to remove double metal cyanide catalysts from polyoxyalkylene polyether polyols prepared therefrom. For example, U.S. Pat. No. 3,427,256 discloses deactivation with strong base followed by reprecipitating, or by treating the product with water or aqueous ammonia and centrifuging. The centrifugation and filtration steps add considerably to the cost of the product, however.
Hinney et al., U.S. Pat. No. 5,248,833 discloses double metal cyanide complex catalyst removal by combining the crude polyol with a C.sub.1-6 aliphatic alcohol and a chelating agent such as ethylene diamine tetraacetic acid (EDTA) to form an insoluble complex, which is then removed via filtration. Heating the crude polyol with alkali metals or alkali metal hydroxides to remove double metal cyanide complex residues is disclosed in U.S. Pat. Nos. 4,355,188 and 4,721,818. Such chemical treatment processes generally destroy or "denature" the catalyst complex. U.S. Pat. No. 5,010,047 discloses dilution of crude polyol with a large amount of non-polar solvent such as hexane or toluene followed by filtration and removal of solvent. U.S. Pat. No. 4,987,271 discloses heating the crude polyol with a pH buffer solution, optionally adding a chelating agent, adding an adsorbent or ion exchange resin, and filtering. All these methods are time consuming, utilize considerable quantities of often expensive reagents, adsorbents, or ion-exchange resins, and are generally energy intensive.